Remove redirect_routes trigger to fill namespace_id

We're using a trigger trigger_sync_redirect_routes_namespace_id to fill namespace_id for redirect_routes. We want to:

• Populate namespace_id within Rails itself
• Remove the trigger and set namespace_id as NOT NULL
• Update subject_key for RedirectRoute to be just using namespace_id

The following discussion from !216123 should be addressed:

• @bmarjanovic started a discussion:

  nit-pick: maybe we can create a follow-up issue to clean-up this when removing the trigger.